Dielectric Relaxation Measurements Research Articles

Does the arrhenius temperature dependence of the Johari-Goldstein relaxation persist above T(g)?

Dielectric spectra of the polyalcohols sorbitol and xylitol were measured under isobaric pressures up to 1.8 GPa. At elevated pressure, the separation between the alpha and beta relaxation peaks is larger than at ambient pressure, enabling the beta relaxation times to be unambiguously determined. Taking advantage of this, we show that the Arrhenius temperature dependence of the beta relaxation time does not persist for temperatures above T(g). This result, consistent with inferences drawn from dielectric relaxation measurements at ambient pressure, is obtained directly, without the usual problematic deconvolution the beta and alpha processes.

Changes in dynamic crossover with temperature and pressure in glass-forming diethyl phthalate.

Dielectric relaxation measurements have been used to study the crossover in dynamics with temperature and pressure, onset of breakdown of the Debye-Stokes-Einstein law, and the relation between the alpha and the beta relaxations in diethyl phthalate. The measurements made over 10 decades in frequency and a broad range of temperature and pressure enable the dc conductivity and the alpha- and the beta-relaxations to be studied altogether. The isobaric data show that the alpha-relaxation time tau(alpha) has temperature dependence that crosses over from one Vogel-Fulcher-Tammann-Hesse form to another at T(B) approximately 227 K and tau(alpha) approximately 10(-2) s. The dc conductivity sigma exhibits similar crossover at the same T(B). At temperatures above T(B), tau(alpha) and sigma have the same temperature dependence, but below T(B) they become different and the Debye-Stokes-Einstein law breaks down. The breadth of the alpha relaxation is nearly constant for T<T(B), but decreases with increasing temperature for T>T(B). The time dependence of tau(beta) is Arrhenius, which when extrapolated to higher temperatures intersects tau(alpha) at T(beta) nearly coincident with T(B). Isothermal measurements at various applied pressures when compared with isobaric data show that the shape of the alpha-relaxation depends only on tau(alpha), and not on the T and P combinations. At a constant temperature, while tau(alpha) increases rapidly with pressure, the beta-relaxation time tau(beta) is insensitive to applied pressure. This behavior is exactly the same as found in 1,1(')-bis (p-methoxyphenyl) cyclohexane. The findings are discussed in the framework of the coupling model.

Inference of the Evolution from Caged Dynamics to Cooperative Relaxation in Glass-Formers from Dielectric Relaxation Data

Isothermal dielectric relaxation measurements were carried out on xylitol and threitol. These, together with existing data on sorbitol and glycerol, enabled us to identify the general features in t...

Dielectric relaxation study of Pb1−xLaxMoO4+δ (x = 0–0.3)oxide-ion conductors

DC conductivity and dielectric relaxation measurements are exploited to study the influenceof La substitution on the dielectric properties and oxygen-ion transportation in PbMoO4samples. The DC conductivity of La-doped samples is about 10−3 S cm−1around 1073 K. A dielectric loss peak with activation energy of 0.6–0.8 eV is observed inthe temperature spectrum as well as in the frequency spectrum for all La-doped PbMoO4samples. With increasing La doping content, this peak becomes higherand shifts to higher temperature or lower frequency, and the activationenergy becomes larger. It is suggested that this dielectric loss peak isassociated with the short-distance diffusion of oxygen ions (or oxygenvacancies) between the 16f and 8e sites of the scheelite structure type withI41/asymmetry.

DTA and Dielectric Studies of a Substance with the Nematic, Smectic A, and Smectic C Polymorphism at Ambient and Elevated Pressures

For the first time the low frequency relaxation process in two smectic phases (smectic A and smectic C) was studied at elevated pressures with the aid of DTA and dielectric spectroscopy. The substance studied, 2-(4-hexyloxyphenyl)-5-octyl-pyrimidine (6OPB8 in short) exhibits the nematic (N) - SA - SC phase sequence. The p-T phase diagram was established with DTA. However, the SA - SC transition was not observed in the DTA, but could be detected by dielectric relaxation measurements. The dielectric relaxation time measured as function of temperature and pressure, τ∥(p,T), enabled us to calculate the activation volume, Δ#V =RT(∂lnτ /∂p)T, and activation enthalpy, Δ#H = R(∂lnτ /∂T−1)p. It was found that Δ#V(SA) &gt; Δ#V(SC) and Δ#H(N) ≫ Δ#H(SA) &gt; Δ#H(SC), indicating that the molecular rotations around the short axes are more feasible in the tilted SC than in the orthogonal SA phase.

Microwave Dielectric Properties of Solid and Liquid Foods Investigated by Time‐domain Reflectometry

ABSTRACT: Microwave dielectric relaxation measurements by a time‐domain reflectometry (TDR) method were performed on solid and liquid foods, covering the frequency range between 100 kHz and 10 GHz. Carrots, radishes, potatoes, rice, milk, cheese, egg whites, egg yolks, fish, chicken, agar aqueous gel, gelatin aqueous gel, and ovalbu‐min aqueous gel were examined. The relaxation processes caused by the reorientation of free water and bound water, interfacial polarization, and polymer chain motion were discussed. Whiskey, beer, and wine also were examined by TDR, and a relaxation process caused by reorientation of molecular dipoles in water and alcohol was observed. The obtained dielectric parameters for beverages were compared with those of ethanol‐water mixtures.

A combined molecular dynamics simulation, experimental and coupling modelstudy of the ion dynamics in glassy ionic conductors

Dielectric relaxation measurements of the ion dynamics in glassy, molten andcrystalline ionic conductors show general properties independent of chemicalcomposition and structure. These include the appearance of a near-constant lossat high frequencies/low temperatures and the transition to a many-particleion-hopping regime at lower frequencies. We use a combination of moleculardynamics simulation, experimental data analysis and the coupling model tocharacterize the ion dynamics in the near-constant-loss regime, the transitionzone, and the many-particle ion-hopping regime. An improved understanding isgained of the origin of the near-constant loss and the evolution of the iondynamics, from short times when the ions are caged, to long times when they areno longer caged but participate in the many-particle dynamics, giving risefinally to dc conductivity. Reasons are given to refute criticism of use ofthe electric modulus to represent and interpret experimental data.

DTA, X-ray and dielectric relaxation studies of 14CB at atmospheric and elevated pressures

The pressure-temperature phase diagram of 4′-tetradecyl-4-cyanobiphenyl (14CB) up to 220 MPa (2.2 kbar) and between 320–400 K was established using DTA. The temperature range of the smectic A (SmA) phase slightly increases with pressure. The layer spacing d at 1 atm was determined as a function of temperature using X-ray diffraction. It was related to the molecular length l by the ratio d/l ˜ 1.4. The dielectric relaxation measurements in the isotropic and smectic Ad phases of 14CB at 1 atm were performed in the frequency range 10 kHz-3 GHz. Contributions from both principal rotational motions, i.e. around the short and long molecular axes, were separated. The relaxation measurements under high pressure in the SmA phase covered the low frequency process. The longitudinal relaxation time τ1, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature dependences, giving activation volumes, Δ# V = RT (∂ ln τ1 / ∂p)T, and activation enthalpies, Δ# H = RT(∂ ln τ1 / ∂T -1)p, respectively. Surprisingly, all the activation quantities characterizing the rotational motions of 14CB molecules under different conditions are nearly the same as those determined recently for the much shorter homologue, 8CB. This indicates that the 14CB molecule is in fact relatively short due to conformational motions of the alkyl tail.

Dielectric and Anelastic Relaxation in PMN-PT Relaxors

Measurements of dielectric and anelastic relaxation have been performed on ferroelectric relaxor PMN-10%PT. Both the dielectric and elastic susceptibilities presented the typical relaxor peak, although much less intense in the elastic case. The anelastic measurements have been extended down to 1 K. finding persistence of the acoustic losses down to liquid helium temperatures. This suggests the existence of isolated or loosely interacting ions that remain mobile within double-well potentials with particularly low barriers also when the polar clusters are completely frozen. The imaginary part of the elastic compliance could be fitted with a temperature independent distribution function of the barriers, assuming that relaxation occurs between inequivalent states, with asymmetries of the order of 100 K or more.

Dielectric relaxation study of pyridine-alcohol mixtures using time domain reflectometry

Time domain reflectometry is applied to dielectric relaxation measurements on pyridine-alcohol (methanol, ethanol, propan- 1-o1 and butan-1 -o1) mixtures over the entire concentration range and over the frequency range 10MHz-10GHz at 5, 15, 25 and 35°C. From the data, static permittivity and the dielectric relaxation time are extracted using a bilinear calibration method and a nonlinear least-squares fit method. These mixtures exhibit a principal dispersion of the Debye type at microwave frequencies. The Kirkwood correlation factor, which contains information regarding solute-solvent interaction and corresponding structural information, was obtained for these systems, and the excess inverse relaxation time and molar energy of activation for all these systems were determined. The values of the static permittivity, the relaxation time and the Kirkwood correlation factor decrease with increased pyridine concentration in alcohol. The static permittivity of the mixtures fits the modified Brugemann model well.

Pronounced Dielectric Relaxation Behavior of a Small Ionic Species, p-Toluenesulfonate, in Aqueous Solution

Aqueous solutions of a small size electrolyte, sodium p-toluenesulfonate (NapTS), were examined for dielectric relaxation behavior. Dissociated pTS- shows pronounced dielectric relaxation behavior in the aqueous solutions well-described with a Debye-type relaxation function. The value of the dielectric relaxation time is much longer than that of water molecules and is very close to the rotational relaxation time determined from the value of a spin−lattice relaxation time (T1, 1H NMR). This means that pTS- keeps significantly large dipole moment (μ) and its rotational relaxation mode is detected by the dielectric relaxation measurements. Moreover, the concentration dependence of dielectric relaxation data suggests that 2−3.5 water molecules are tightly hydrated to pTS- in aqueous solution. We also carried out ab initio quantum chemical calculations to obtain the optimized geometries of individual pTS-, pTS- dihydrate, and pTS- trihydrate. The magnitudes of μ evaluated to be 7.3, 6.1, and 6.2 D, respectively, at the optimized geometries are significantly large and are crucial evidence that pTS- keeps large μ in aqueous solution.

Dynamics of Sorbitol at Elevated Pressure

Dielectric relaxation measurements were carried out on sorbitol as a function of pressure at various temperatures. The almost linear dependence of the structural relaxation times on pressure yields values for the activation volume. In light of results for xylitol and glycerol, the activation volume is found to be an increasing function of molecular size. Because the pressure coefficients of the glass temperature for these polyalcohols are all equal, their fragilities should parallel their respective activation volumes. This expectation is borne out by experimental measurements at ambient pressure. Analysis of the volume dependence of the relaxation times reveals that temperature, rather than density, dominates the structural relaxation of sorbitol. At frequencies higher than the structural relaxation, a secondary process is observed. The weaker pressure sensitivity of the latter effects better resolution of the two peaks at high pressures. The relaxation time for the secondary process is consistent with a calculation from the coupling model based on identification of the secondary process as the precursor to the highly cooperative structural relaxation.

Thermally induced coupling of phase separation and gelation in an aqueous solution of hydroxypropylmethylcellulose (HPMC)

Thermally induced coupling of gelation and phase separation in polysaccharide aqueous solutions has a complex feature because of critical and tricritical phenomena, thermally induced hydrophobic interaction, and molecular-weight distribution of the polysaccharide. To elucidate the process, the criticality of a hydroxypropylmethylcellulose (HPMC) aqueous solution was assessed, and then dielectric relaxation and fluorescence intensity experiments were carried out. The diffusion coefficient of the solution with a weight fraction of HPMC being 0.06 could be extrapolated to zero at the cloud-point curve which showed the criticality of the solution. The fluorescence intensity increased at a temperature much lower than the cloud point and the gel point, especially for concentrated solutions, indicating the hydrophobic interaction as the driving force of the gelation coupled by the phase separation. Dielectric relaxation measurements by time-domain reflectometry revealed two characteristic relaxations of chain motions around 100 MHz and orientation of free water around 20 GHz , which is accompanied by a low-frequency tail reflecting hydration water.

Comparison of the effect of reactive and non-reactive steric stabilisers on the mechanism of film formation in methyl methacrylate/butyl acrylate copolymers latexes. Part 2. Electrical conduction and dielectric spectroscopic investigations

A comparison of the film forming ability of methyl methacrylate (MMA)/butyl acrylate (BA) latex copolymers stabilised by either a reactive or a non-reactive steric stabiliser is reported and indicates the key role of the degree of polymerisation of the hydrophilic ethylene oxide chain on the coalescence process. The study uses dielectric measurements to follow and identify the various stages of film formation. The ability of the stabiliser to segregate and diffuse from the interfacial layer into the surrounding media influences in both the rate of the coalescence process and the final physical properties of the films formed. Dielectric relaxation and electrical conductivity measurements indicate the mechanistic complexity of the coalescence process and illustrate that the clear differences exist between the behaviour of the two systems investigated. A model for the film formation processes is presented which incorporates the various features identified by the dielectric study.

Investigation of the correlation between structural relaxation time and configurational entropy under high pressure in a chlorinated biphenyl

Dielectric relaxation measurements on a chlorinated biphenyl (PCB62) were carried out over a broad frequency range, with variation of both temperature and pressure. In combination with calorimetric determinations of the configurational entropy, these data could be described using the Adam–Gibbs model. Specifically, the experimental results were interpreted using a recently introduced equation for both the temperature and pressure dependencies of the structural relaxation time. The τ(T,P) data for PCB62 yielded values of the fitting parameters consistent with known physical properties of the material. A change of the dynamics was evident in isobaric measurements at atmospheric pressure, corresponding to a value of the relaxation time τB∼5×10−5 s. A related change of dynamics was observed in isothermal experiments at varying pressures. It is noteworthy that the latter transpired at a very similar τ∼τB. Moreover, the shape of the relaxation function depended only on the value of τ. We believe this is the first reported evidence of such a change of dynamics in experiments using pressure as a variable. These results suggest that this change is governed by the time scale of the relaxation, independently of any particular combination of T and P.

Dynamic light scattering studies of supercooled phenylphthalein–dimethylether dynamics under high pressure

The behavior of the α-relaxation process in phenylphthalein–dimethylether (PDE) under high pressure was studied by means of dynamic light scattering–photon correlation spectroscopy (DLS-PCS). The temperature dependence of the relaxation time of the α-process at ambient pressure was found to resemble that obtained from dielectric relaxation (DR) measurements. On the basis of the DLS measurements one can conclude that PDE satisfies the correlation between nonexponentiality and fragility established for low-molecular weight glass forming liquids and polymers. On the other hand, dielectric relaxation studies indicate that PDE is an exception from this correlation. These differences between the DLS and DR studies do not result from the overlap of the α- and β-processes in the DLS experiments but might be due to the different coupling of the probes seen in these experiments (optical anisotropy and dipole moment) to the dynamics of the entire PDE molecule. We also studied the effect of pressure on fragility and the glass transition temperature in PDE and we found that fragility is practically independent of pressure.

Effect of nano-porous Al 2O 3 on thermal, dielectric and transport properties of the (PEO) 9LiTFSI polymer electrolyte system

Thermal, electrical conductivity and dielectric relaxation measurements have been performed on (PEO) 9LiTFSI+10 wt.% Al 2O 3 nano-porous polymer electrolyte system. It is observed that the conductivity enhances substantially due to the presence of the filler particles with different surface groups. The highest enhancement is found for the filler particles with acidic groups followed by basic, neutral, and weakly acidic. The results reveal that the filler particles do not interact directly with poly(ethelene) oxide (PEO) chains indicating that the main chain dynamics governing the ionic transport has not significantly affected due to the filler. The results are consistent with the idea that the conductivity enhancement is due to the creation of additional sites and favourable conduction pathways for ionic transport through Lewis acid–base type interactions between the filler surface groups and the ionic species. This is reflected as an increase in the mobility rather than an increase in the number of charge carriers. A qualitative model has been proposed to explain the results.

Phase Diagram and Dielectric Relaxation Studies of n-Hexyl-isothiocyanato-biphenyl (6BT) in the Smectic E Phase under High Pressure

The pressure-temperature phase diagram of n-hexyl-isothiocyanato-biphenyl (6BT) at up to 190 MPa and 250 - 400 K was established with the aid of DTA. The dielectric relaxation measurements in the SmE phase of 6BT were performed in the pressure range of 0.1 - 150 MPa and the temperature range of 320 - 350 K. The Debye-type relaxation process was observed in the frequency range of 100 Hz - 100 kHz. The longitudinal relaxation time characterizing the molecular reorientations around the short axis was analyzed with respect to the pressure and temperature dependencies, yielding the activation volume, Δ#V = RΤ(∂ ln τ/∂p)Τ , and activation enthalpy, Δ#H = R(∂ ln τ/∂Τ-1)p , respectively. The results are compared with the analogous data obtained recently for 8BT and other similar compounds having the nematic and SmA liquid crystalline phases.in the telluride retain their molecular character, with small intercluster interactions.

Relaxation processes of off-center impurities in KTN:Li crystals

The dielectric and ferroelectric properties of K 1− x Li x Ta 1− y Nb y O 3 (KLTN) crystals as functions of temperature, frequency, and electric field are presented. Measurement of the polarization as a function of temperature indicates the existence of polarized microregions at temperatures well above the phase transition (PT) temperature. In the vicinity of the PT those microregions respond cooperatively and contribute to the observed macroscopic polarization. This cooperative behavior was also evident from dielectric relaxation measurements. Two relaxation processes were observed, one at high frequency and the other at low frequency. In both processes the relaxation time and the relaxation step ( Δε) were found to increase as the PT temperature was approached. Those processes originate from movement of off-center ions in a multi-well potential. In the vicinity of the PT the correlation length of the host lattice is increased and the movements of the off-center ions become more and more correlated. The increased number of cooperatively relaxing ions increases the relaxation time and the relaxation amplitude.

Optimization of tunable laser glasses with the aid of dielectric relaxation and adsorption measurements

The recently developed tunable dye lasers in the visible are obtained by incorporation of stable laser dyes into glasses prepared by the sol–gel method. In order that the lasers be photostable and more efficient it is essential that the dye molecules penetrate into the glass matrices in a unimolecular form and be protected from the surroundings. In this respect, the size of the pores and their distribution in the glass are of major importance. In this paper the influence of the various catalysts on the porosity of the glasses is presented. The pore sizes, their surface area, pore volume, pore size distribution and glass effective density are presented, and the solid state tunable lasers prepared under the optimal conditions are demonstrated. The results obtained by the conventional adsorption methods (water, nitrogen adsorption) and mercury porosimetry are compared with dielectric measurements and the conclusion is that the latter provide information about charge movement in glasses with the smallest pores. The water vapor from the atmosphere is adsorbed within the smallest pores in a monolayer resulting in the formation of dipoles, that can be detected by dielectric measurements.